Dipped latex products such as gloves, condoms, balloons and finger cots are made by well-established manufacturing process where the curing agent typically sulphur, accelerators, activators, surfactants are added to one or more of the lattices, namely: Natural Rubber Lattices including Hevea brasiliensis and Guayule latex and Synthetic Rubber Lattices including Nitrile, Polychloroprene and Synthetic Polyisoprene to prepare a liquid compound used in the dipping process. Dipping is a process where formers, which are articles having the shape of the required finished articles such as gloves, condoms, balloons, etc., are dipped into a liquid compound of the selected latex. When the formers are withdrawn from the compounded latex, a thin film of the latex compound remains coated on the formers. Then the film is dried and heated to cure the polymer to form the dipped article.
Often successive layers are applied by carrying out repeated dipping of the coated formers into latex compound again and then drying the film. The process is repeated on the dried film to improve physical properties of the finished product and/or add colour to the film. Two or more repetitions of dipping is carried out depending upon the nature of Formulation of compound, intended level of thickness and physical properties, colour addition to the coating of the film etc. When the drying and curing processes are completed the coated film is removed from the former in the shape of the required article. In large scale manufacturing, the dipping process is automated so that a number of formers are mounted on an endless conveyor chain that carries the formers through stages of immersion into latex compound (termed dipping) followed by drying in heated ovens and eventually removal of article from the formers.
In conventional continuous dipping processes, all the compounding ingredients and curing agents, typically sulphur, accelerators, activators, and surfactants, are added to lattices in the manufacturing stage called compounding. As per the state of the art, all the compounding ingredients must be mixed homogenously in the lattices to produce the dipped articles with good physical properties.
The majority of the dipped latex products manufacturers use the dipping process broadly described above. The overall machinery inclusive of latex compound storing vessels and related automation to carry out the dipping process is called a dipping plant. The latex holding vessels in which the latex compound is held to enable formers to be dipped are called dippers or dipping tanks. These dippers have built-in cooling provisions to maintain latex temperature within pre-determined range. Depending upon plant design and process requirements the plant would have two or more dippers and associated drying ovens. For example, if two coatings of latex compound are to be made on the former then there could be two different dippers. In some designs the same formers are repeatedly immersed into the dippers twice. This is achieved by routing the formers carried by endless conveyor chain twice into the same dipper. Many variations on multiple dips are in use in the industry.
Condoms, which are manufactured from the rubber lattices by employing dipping process, are commonly used during sexual intercourse to reduce the probability of pregnancy and/or prevent spreading sexually transmitted diseases. Traditionally condoms have been made from natural rubber latex but more recently condoms made from various synthetic elastomers have been introduced. In particular, condoms made from synthetically prepared polyisoprene have been found to offer a number of advantages over traditional condoms.
The synthetically prepared polyisoprene serves as a suitable substitute of natural latex for manufacturing condoms. However, the synthetic polyisoprene is known to cure very fast. This pre-disposes manufacturing process to be designed such that the cure rate is kept well controlled until conversion into condoms by dipping process. The fast curing is managed through lowering the temperature of the latex in the plant. As compared to natural rubber latex where the compound is maintained at about 25° C. the synthetic latex needs to be maintained at 15° C. Therefore, while manufacturing dipped latex articles from the synthetic lattices the temperature is required to be maintained at 15° C. so that reduction in the rate of curing (in this case pre-vulcanisation) can be achieved.
One of the methods disclosed in Patent publication WO2007113463 requires the latex to be stored and used in low temperature preferably at 15° C. or less to slow down curing. This process of cooling is expensive.
In another method of preventing the fast cure is disclosed in Patent publication U.S. Ser. No. 10/085,890. That patent requires the use of unusual compounding ingredients some of which are considered toxic.
For the purpose of preparing dipped latex products both natural latex and synthetic latex are used. Both types of lattices have inherent disadvantages due to pre-vulcanization. Therefore, there exists a need for a process that can eliminate the influence of any level of pre-vulcanization on the outcome of the dipping process and facilitate manufacturing of dipped latex products from the latex unencumbered from its inherent disadvantages.